In today's world, the widespread use of wired and wireless communication technology, such as the Internet, broadcasting, etc., has made it very important to protect and secure communication networks and individual terminals. In hopes of finding the ultimate solution to security issues, experts in the field are currently researching quantum cryptography technology whereby security is absolutely guaranteed based on principles of quantum mechanics rather than conventional complex mathematical calculations.
Protocols used in quantum cryptography technology include BB84 protocol, named after its inventors Charles Bennett and Gilles Brassard, E91 protocol using an Einstein-Podolsky-Rosen (EPR) state, B92 protocol developed by and named after C. H. Bennett in 1992, and so on. Among the protocols, the commonly used BB84 protocol, which uses the polarization state of a single photon, will now be described with reference to FIG. 1.
As illustrated in FIG. 1, BB84 protocol using the polarization state of a single photon uses 4 polarization states constituting 2 bases. In other words, a transmitter, Alice, randomly selects one of the two bases and randomly selects and transmits one of two quantum states 0 and 1, i.e., cryptographic key values, of the selected basis, to a receiver, Bob. The receiver who has received the quantum state also randomly selects one of the two bases and measures the received quantum state using the selected basis. After the receiver performs the measurement, the transmitter and the receiver tell each other the bases that they have randomly selected. Here, when the transmitter has selected the same basis as the receiver, the result measured by the receiver is the same as the quantum state randomly selected by the transmitter, and thus the two users have the same cryptographic key.
Meanwhile, a method using double asymmetric Mach-Zehnder interferometers (U.S. Pat. No. 5,307,410) is employed in conventional quantum cryptography systems. The method is simple but when a quantum cryptography system is constructed using a long-distance optical fiber between two users separated far apart from each other, polarization drift of an optical pulse signal passed through the optical fiber and phase drift of the interferometers must be compensated for. According to the method, it is possible to actively compensate for the polarization and phase drift, but it is expensive and impractical.
Meanwhile, a recently invented quantum cryptography technique (“Automated ‘Plug & Play’ Quantum Key Distribution,” Electron Lett. 1998) automatically compensates for polarization drift of an optical pulse signal passed through an optical fiber and phase drift of an interferometer by transmitting the optical pulse signal in a round-trip using a Faraday mirror. However, such an automatically compensating quantum cryptography technique performs round-trip transmission, thus having a limited transmission rate due to Rayleigh Backscattering, whereby when an optical pulse signal propagates through an optical fiber, a part of the light is captured in the fiber and propagated in the backward direction.